The present invention relates generally to a method of fabricating a large area multi-element contactor and, more particularly, to a segmented contactor fabricated by mounting multiple contactor units on a substrate.
Semiconductor devices (such as integrated circuits) are generally fabricated on a substrate of silicon known as a wafer. A single wafer typically includes a large number of devices (such as integrated circuits) that are grouped into units called dies. A single wafer typically has a plurality of dies formed thereon. Each die is later singulated from the wafer and further processed and packaged. Modern technology typically uses 8-inch (200-mm) diameter wafers, and is moving to 12-inch (300-mm) wafers. Essentially every single device fabricated on a wafer needs to be electrically tested by probing. Probing more than one device at a time is particularly advantageous. Modern probing equipment can probe 32 or more semiconductor devices at the same time. However, this is often only a small fraction of the total number of devices on a wafer. There has been great interest in developing a probing system that can contact more, preferably all devices on a wafer at the same time.
It is generally desirable to identify which of the plurality of dies on a wafer are good prior to their packaging, and preferably prior to their being singulated from the wafer. To this end, a wafer xe2x80x9ctesterxe2x80x9d or xe2x80x9cproberxe2x80x9d may be employed to make a plurality of discrete pressure connections to a like plurality of discrete connection pads (bond pads) on the dies. In this manner, the semiconductor dies can be tested, prior to singulating the dies from the wafer.
Typically, semiconductor devices are exercised (burned-in and tested) only after they have been singulated (separated) from the wafer and have gone through another long series of xe2x80x9cback-endxe2x80x9d process steps in which they are assembled into their final xe2x80x9cpackagedxe2x80x9d form. The added time and expense of singulating and packaging the device goes to waste if the final xe2x80x9cpackagedxe2x80x9d device is found to be defective after packaging. Consequently, performing testing or burn-in of semiconductor devices prior to their being singulated from the wafer has been the object of prolonged endeavor.
Modern integrated circuits include many thousands of transistor elements, for example, with many hundreds of bond pads disposed in close proximity to one another; e.g., 4 mils (about 100xcexc) center-to-center. One common layout of the bond pads has one or more rows of bond pads disposed close to the peripheral edges of the die. Another common layout has is called xe2x80x9clead on centerxe2x80x9d (LOC) with typically a single row of contacts along a center line of a die. Other layouts, some irregular, are not uncommon. The proximity and number of pads is a challenge to the technology of probing devices.
Generally, probing devices for testing semiconductor devices on a wafer have involved providing a single test substrate with a plurality of contact elements for contacting corresponding pads on the wafer being tested. To test a full wafer simultaneously generally requires extremely complex interconnection substrates, which may easily include tens of thousands of such contact elements. As an example, an 8-inch wafer may contain 500 16 Mb DRAMs, each having 60 bond pads, for a total of 30,000 connections between the wafer under test (WUT) and the test electronics. Earlier solutions included mating with some subset of these connections to support limited or specialized testing. It would be advantageous to fully connect an entire wafer.
Moreover, the fine pitch requirements of modern semiconductor devices require extremely high tolerances to be maintained when bringing the test substrate together with the wafer being tested. During testing or burn-in processes, heat is produced which causes thermal expansion of the underlying substrate materials. Thermal expansion presents a further challenge to connecting a test substrate to the WUT because of the extremely tight tolerances and close spacing of pads.
To effect reliable pressure connections between contact elements and, e.g., a semiconductor device, one must be concerned with several parameters including, but not limited to: alignment, probe force, overdrive, contact force, balanced contact force, scrub, contact resistance, and planarization. A general discussion of these parameters may be found in U.S. Pat. No. 4,837,622, entitled xe2x80x9cHigh Density Probe Card,xe2x80x9d incorporated by reference herein, which discloses a high density epoxy ring probe card including a unitary printed circuit board having a central opening adapted to receive a preformed epoxy ring array of probe elements.
A more sophisticated probe card uses resilient spring elements to make contact with a device on a wafer. Commonly assigned U.S. Pat. No. 5,806,181, entitled xe2x80x9cContact Carriers for Populating Larger Substrates with Spring Contacts,xe2x80x9d issued Sep. 15, 1998, (""181 patent), incorporated by reference herein, discloses such a probe card. The resilient spring elements of the ""181 patent are pre-fabricated on individual spring contact carriers (xe2x80x9ctilesxe2x80x9d).
The resilient spring elements can alternatively be prefabricated on the wafer itself. This configuration is known as MOST Technology, using Microspring Contacts On Silicon. Such a wafer is conveniently manufactured using techniques described in commonly assigned, copending U.S. patent application Ser. No. 08/558,332, entitled xe2x80x9cMethod of Mounting Resilient Contact Structures to Semiconductor Devices,xe2x80x9d filed Nov. 15, 1995, incorporated by reference herein. A contactor or testing substrate that can perform a wafer-level test or burn-in procedure on a MOST wafer must provide corresponding conductive areas that can precisely align with the thousands of microsprings disposed on the wafer.
Providing a contactor that can be precisely aligned with each of the resilient spring elements or bond pads is challenging because of tolerances and the expansion of the underlying substrate materials due to heat produced during the testing or burn-in processes. Also, providing a large size contactor that has corresponding conductive areas for each spring element on the wafer under test can be problematic because if one of the thousands of conductive areas is defective, the entire contactor will be deemed defective.
Thus, what is needed is a segmented contactor that provides separate contactor units for performing wafer-level testing or burn-in procedures and that minimizes problems related to tolerances and thermal expansion.
In one example of the present invention, a segmented contactor comprises a relatively large backing substrate and at least one relatively small contactor unit (xe2x80x9ctilexe2x80x9d) mounted to the backing substrate. Preferably, a plurality of contactor units is provided. The contactor units are disposed on the front (facing the WUT or other device) surface of the backing (support) substrate. It is also possible (and may be preferable) that one contactor unit is bigger than an individual device under test (DUT) and xe2x80x9cmatesxe2x80x9d with two or more DUTs.
The contactor units can include active semiconductor devices, such as application-specific integrated circuits (ASICs). For example, the ASIC can enable the number of signals being provided to the test substrate from an outside source (e.g., a host controller) to be minimized.
In one example of the invention, resilient contact elements that provide the conductive pressure connections are preferably mounted by their bases directly to the WUT (i.e., to the DUTs on the WUT) so as to have free ends extending to a common plane above the surface of the WUT. The segmented contactor of the present invention preferably has a coefficient of thermal expansion which is well-matched with that of the WUT. Alternatively, the resilient (or spring) contact elements are mounted to the contactor units of the segmented contactor.
An example of a method of fabricating a segmented contactor is provided wherein a plurality of contactor units is mounted on a backing substrate such that resilient contact elements attached to a device on a silicon wafer can be aligned with a plurality of conductive contact areas on each contactor unit during testing.
An exemplary method includes forming a plurality of contactor units on a single contactor substrate, testing electrically each of the contactor units, separating each of the contactor units from the single contactor substrate, and assembling the contactor units which have passed the electrical testing to form the segmented contactor.
Preferably, the contactor units are tested before being separated from the single contactor substrate onto which they are formed. Alternatively, the contactor units can be tested individually after being separated.
Also, each contactor unit preferably includes a plurality of electrically conductive leads extending horizontally beyond an edge of each contactor unit. The plurality of leads is preferably in the form of a flex strip which can have a connector attached to the leads for connecting the contactor unit to an external testing device.
Assembling the contactor units to form the segmented contactor can include providing an assembly fixture for holding the contactor units during the assembly. An example of an assembly fixture is a plate that defines holding spaces. A contactor unit is placed into a corresponding holding space on the plate. Each contactor unit has a first side and a second side. An adhesive or attachment means can be provided on the second side either before or after the contactor unit is placed within its respective holding space on the plate. After the contactor units are placed into respective holding spaces, a backing substrate is pressed onto the adhesive to mount the contactor units to the backing substrate. The backing substrate is then lifted away from the plate. The contactor units are thus properly aligned and mounted to the backing substrate.
The assembly fixture provided for the assembly of the segmented contactor is preferably a flat plate that includes grooves into which guide blocks are placed to define the holding spaces between the guide blocks. The guide blocks provide the proper relative alignment of each contactor unit.
The method and apparatus of an example of the present invention also provide that the first sides of the contactor units are substantially coplanar when mounted onto the backing substrate.
The contactor units can be removably mounted to the backing substrate, such that each contactor unit can be removed and replaced upon failure or discovery of a defect in any one particular contactor unit, for example.
An example of the device of the present invention can be readily used for partial to full wafer-level testing of devices which have spring contact elements mounted thereto. In use, the segmented contactor including the backing substrate with plurality of contactor units mounted thereto and having conductive leads extending therefrom (the leads being connected to external testing equipment) is urged toward the wafer under test so that the resilient contact elements extending from the devices on the wafer make contact with corresponding conductive areas or pads on corresponding contactor units of the segmented contactor. The ability of all the resilient contacts to make contact with the plurality of contactor units, all at once, can facilitate such processes as wafer-level burn-in or testing. However, it is not necessary that every die on the wafer contact a corresponding contactor unit on the segmented contactor at once.
An alternative example of the present invention includes a segmented contactor which includes spring contact elements mounted to the contactor units of the segmented contactor.
It will also be appreciated that a segmented contactor of the invention may be used, after assembly, to test devices other than a semiconductor wafer, such as another contactor or a printed circuit board.